Radiation-emitting devices are generally known and used for radiation therapy in the treatment of patients, for example. Typically, a radiation therapy device includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. A linear accelerator is located in the gantry for generating a high-energy radiation beam for therapy. This high radiation beam can be an electron radiation or photon (X-ray) beam. During treatment, the radiation beam is provided on one zone of a patient lying in the isometer of gantry rotation.
The goal of radiation treatment planning is to maximize the dose to the target volume while protecting radiation sensitive healthy tissue. The X-ray bean intensity often varies over the treatment field by placing an X-ray absorber in the beam's path. This allows the target volume to be placed in regions of high beam intensity, while the surrounding radiation sensitive tissue is protected by placement in low intensity regions. A simple example is a wedge-shaped isodose distribution, which has been found to be clinically useful in treatment plans.
One frequently used method is to place a physical wedge accessory (i.e., a wedge-shaped absorber) in the X-ray beam path that exponentially decreases the beam intensity laterally across the treatment field. A desirable wedge-shaped isodose distribution results. The "toe" of the wedge (i.e., where the thickness of the wedge is the smallest) produces the high beam intensity region, since this portion of the beam has the least attenuation.
The use of the physical wedge accessory has some negative side effects, however. The primary beam intensity is reduced at the target volume; thus, treatment times are increased. Further, scattering of the beam outside the treatment field causes additional dose to be delivered outside the target volume. It also introduces a spatial energy dependence (i.e., hardness) to the beam, affecting the depth at which the radiation is absorbed across the treatment field. Additional time and effort are required to design, validate, manufacture, install/remove, and store the accessories. In addition, only a limited number of wedge angles are available.
The virtual wedge function integrated into some treatment devices, such as MEVATRON and PRIMUS systems from Siemens Corporation, New Jersey, is used to achieve an accumulated dose profile and isodose distribution similar to that of a physical wedge accessory. The virtual wedge function is accomplished by controlling the travel of a secondary collimator jaw and the X-ray beam intensity during irradiation. The virtual wedge scheme eliminates most of the problems associated with the physical wedge.
A further feature of radiation therapy involves portal images, which are commonly used in radiation therapy to verify and record the patient tumor location. Portal images include manual (film) and electronic images (EPI) taken before or after the treatment. Electronic portal images (EPI), when taken before the treatment, give the therapist the opportunity of correcting for minor patient positioning errors before treatment. Further, EPI allows therapists to take images remotely without going inside the treatment room.
Conventional radiation therapy technique typically utilize field sizes in the range of 5 to 30 cm, and doses per fraction in the range of 70 to 200 MU (monitor units). With portal images taken once a week and requiring up to 10 MU each, the portal image dose does not introduce a significant impact on the original dosimetric calculations. However, with intensity modulation techniques (IMRT), the treatment fields and doses are calculated to be more geometrically and dosimetrically precise. Treatment fields are decomposed into dose segments, which are smaller than in conventional treatments. When fields are small, dosimetric output factors are reduced as well. Therefore, portal image doses can affect the original dosimetric plan of IMRT.
When the treatment technique is fixed and using EPI, the problem can be resolved by including the dosimetric effect of the portal image into the initial dosimetric calculations at the treatment planning level. For virtual wedge treatment, the dynamic nature of the therapy restricts the use of portal images in assisting repositioning of the patient before treatment.
In accordance with known, prior art virtual wedge treatments, when programming the field size, all jaws are moved to their preset positions, as shown in FIG. 1a. When the treatment is accepted by a therapist, the jaw 30 acting as the dynamic jaw closes to a minimum gap position, usually approximately 0.5 centimeters (cm) from the stationary or opposite jaw 32, as shown in FIG. 1b. The dynamic jaw 30 then automatically opens at a constant average speed to the initial gap position, typically about 1 cm from the opposite jaw 32. As the dynamic jaw 30 opens, a pre-treatment jaw calibration speed test is performed, as shown in FIG. 1c, and an interlock is generated if this test fails. Once the dynamic jaw 30 reaches the initial gap position and no interlocks are asserted, the system is ready to begin treatment.
When the therapist starts the treatment, the initial dose is delivered with the dynamic jaw 30 in its initial gap position and is referred to as MU.sub.gap, as shown in FIG. 1d. Once the MU.sub.gap has been delivered, the dynamic jaw 30 is opened at a constant average speed to its final/preset position, while simultaneously the dose rate is varied as a function of the jaw position. FIG. 1e illustrates the dynamic portion with the dose delivered as the jaw 30 travels as MU.sub.trav. The remaining dose is delivered with the dynamic jaw 30 in its idle, final position as shown in FIG. 1f with a dose of MU.sub.idle. While a portal image could be taken during the idle portion of the virtual wedge treatment of FIG. 1f, there would be no chance to correct patient positioning errors, since the idle portion occurs after the majority of the treatment dose has been delivered.
Accordingly, what is needed is a method and system for effectively utilizing portal images in a virtual wedge treatment.